Hollow-Core Photonic Crystal Fiber Based Optical Component for Broadband Radiation Generation

ABSTRACT

Disclosed is an optical component, being configured to function as an optical frequency converter in a broadband radiation source device. The optical component comprises a gas cell, and a hollow-core photonic crystal fiber at least partially enclosed within said gas cell. The local cavity volume of said gas cell, where said hollow-core photonic crystal fiber is enclosed within the gas cell, comprises a maximum value of 36 cm 3  per cm of length of said hollow-core photonic crystal fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 17/485,836, filed on Sep. 27, 2021, which is acontinuation application of U.S. patent application Ser. No. 16/808,141,filed on Mar. 3, 2020 (now U.S. Pat. No. 11,163,208), which claimspriority to European Patent Application 19160457.8, filed on Mar. 4,2019, which are hereby incorporated herein in their entireties byreference.

FIELD

The present invention relates to a hollow-core photonic crystal fiberbased broadband radiation generator, and in particular such a broadbandradiation generator in relation to metrology applications in in themanufacture of integrated circuits.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such process, the resolution formula may be expressed as CD=k₁×λ/NA,where λ is the wavelength of radiation employed, NA is the numericalaperture of the projection optics in the lithographic apparatus, CD isthe “critical dimension” (generally the smallest feature size printed,but in this case half-pitch) and k₁ is an empirical resolution factor.In general, the smaller k₁ the more difficult it becomes to reproducethe pattern on the substrate that resembles the shape and dimensionsplanned by a circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps may be applied to the lithographicprojection apparatus and/or design layout. These include, for example,but not limited to, optimization of NA, customized illumination schemes,use of phase shifting patterning devices, various optimization of thedesign layout such as optical proximity correction (OPC, sometimes alsoreferred to as “optical and process correction”) in the design layout,or other methods generally defined as “resolution enhancementtechniques” (RET). Alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of the pattern at low k1.

Metrology tools are used in many aspects of the IC manufacturingprocess, for example as alignment tools for proper positioning of asubstrate prior to an exposure, leveling tools to measure a surfacetopology of the substrate, for e.g., focus control and scatterometrybased tools for inspecting/measuring the exposed and/or etched productin process control. In each case, a radiation source is required. Forvarious reasons, including measurement robustness and accuracy,broadband or white light radiation sources are increasingly used forsuch metrology applications. It would be desirable to improve on presentdevices for broadband radiation generation.

SUMMARY

In a first aspect of the invention, there is provided an opticalcomponent, being configured to function as an optical frequencyconverter in a broadband radiation source device, comprising: a gascell, and a hollow-core photonic crystal fiber at least partiallyenclosed within said gas cell; wherein, where said hollow-core photoniccrystal fiber is enclosed within said gas cell, and the local cavityvolume of said gas cell comprises a maximum value of 36 cm³ per cm oflength of said hollow-core photonic crystal fiber.

Other aspects of the invention comprise a broadband radiation source andmetrology device comprising the optical component of the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three key technologies to optimizesemiconductor manufacturing;

FIG. 4 depicts a schematic overview of a scatterometry apparatus used asa metrology device, which may comprise a radiation source according toembodiments of the invention;

FIG. 5 depicts a schematic overview of a level sensor apparatus whichmay comprise a radiation source according to embodiments of theinvention;

FIG. 6 depicts a schematic overview of an alignment sensor apparatuswhich may comprise a radiation source according to embodiments of theinvention; and

FIGS. 7(a)-7(c) schematically depicts examples of prior art opticalcomponents in three different configurations;

FIGS. 8(a)-8(c) schematically depicts optical components in threedifferent configurations according to three embodiments of theinvention; and

FIG. 9 schematically depicts a method for lowering the volume of the gascell cavity in a post-processing step according to an embodiment of theinvention.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a mask support (e.g., amask table) MT constructed to support a patterning device (e.g., a mask)MA and connected to a first positioner PM configured to accuratelyposition the patterning device MA in accordance with certain parameters,a substrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA may be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system PS and the substrate W—which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1 ) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, inspection tools (not shown) maybe included in the lithocell LC. If errors are detected, adjustments,for example, may be made to exposures of subsequent substrates or toother processing steps that are to be performed on the substrates W,especially if the inspection is done before other substrates W of thesame batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3 . One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). The key of such “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LAstays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device)—typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

In lithographic processes, it is desirable to make frequentlymeasurements of the structures created, e.g., for process control andverification. Tools to make such measurement are typically calledmetrology tools MT. Different types of metrology tools MT for makingsuch measurements are known, including scanning electron microscopes orvarious forms of scatterometer metrology tools MT. Scatterometers areversatile instruments which allow measurements of the parameters of alithographic process by having a sensor in the pupil or a conjugateplane with the pupil of the objective of the scatterometer, measurementsusually referred as pupil based measurements, or by having the sensor inthe image plane or a plane conjugate with the image plane, in which casethe measurements are usually referred as image or field basedmeasurements. Such scatterometers and the associated measurementtechniques are further described in patent applications US20100328655,US2011102753A1, US20120044470A, US20110249244, US20110026032 orEP1,628,164A, incorporated herein by reference in their entirety.Aforementioned scatterometers may measure gratings using light from softx-ray and visible to near-IR wavelength range.

In a first embodiment, the scatterometer MT is an angular resolvedscatterometer. In such a scatterometer reconstruction methods may beapplied to the measured signal to reconstruct or calculate properties ofthe grating. Such reconstruction may, for example, result fromsimulating interaction of scattered radiation with a mathematical modelof the target structure and comparing the simulation results with thoseof a measurement. Parameters of the mathematical model are adjusteduntil the simulated interaction produces a diffraction pattern similarto that observed from the real target.

In a second embodiment, the scatterometer MT is a spectroscopicscatterometer MT. In such spectroscopic scatterometer MT, the radiationemitted by a radiation source is directed onto the target and thereflected or scattered radiation from the target is directed to aspectrometer detector, which measures a spectrum (i.e. a measurement ofintensity as a function of wavelength) of the specular reflectedradiation. From this data, the structure or profile of the target givingrise to the detected spectrum may be reconstructed, e.g. by RigorousCoupled Wave Analysis and non-linear regression or by comparison with alibrary of simulated spectra.

In a third embodiment, the scatterometer MT is a ellipsometricscatterometer. The ellipsometric scatterometer allows for determiningparameters of a lithographic process by measuring scattered radiationfor each polarization states. Such metrology apparatus emits polarizedlight (such as linear, circular, or elliptic) by using, for example,appropriate polarization filters in the illumination section of themetrology apparatus. A source suitable for the metrology apparatus mayprovide polarized radiation as well. Various embodiments of existingellipsometric scatterometers are described in U.S. patent applicationSer. Nos. 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968,12/922,587, 13/000,229, 13/033,135, 13/533,110 and 13/891,410incorporated herein by reference in their entirety.

A metrology apparatus, such as a scatterometer, is depicted in FIG. 4 .It comprises a broadband (white light) radiation projector 2 whichprojects radiation onto a substrate W. The reflected or scatteredradiation is passed to a spectrometer detector 4, which measures aspectrum 6 (i.e. a measurement of intensity as a function of wavelength)of the specular reflected radiation. From this data, the structure orprofile 8 giving rise to the detected spectrum may be reconstructed byprocessing unit PU, e.g. by Rigorous Coupled Wave Analysis andnon-linear regression or by comparison with a library of simulatedspectra as shown at the bottom of FIG. 3 . In general, for thereconstruction, the general form of the structure is known and someparameters are assumed from knowledge of the process by which thestructure was made, leaving only a few parameters of the structure to bedetermined from the scatterometry data. Such a scatterometer may beconfigured as a normal-incidence scatterometer or an oblique-incidencescatterometer.

Overall measurement quality of a lithographic parameter via measurementof a metrology target is at least partially determined by themeasurement recipe used to measure this lithographic parameter. The term“substrate measurement recipe” may include one or more parameters of themeasurement itself, one or more parameters of the one or more patternsmeasured, or both. For example, if the measurement used in a substratemeasurement recipe is a diffraction-based optical measurement, one ormore of the parameters of the measurement may include the wavelength ofthe radiation, the polarization of the radiation, the incident angle ofradiation relative to the substrate, the orientation of radiationrelative to a pattern on the substrate, etc. One of the criteria toselect a measurement recipe may, for example, be a sensitivity of one ofthe measurement parameters to processing variations. More examples aredescribed in US patent application US2016-0161863 and published USpatent application US 2016/0370717A1 incorporated herein by reference inits entirety.

Another type of metrology tool used in IC manufacture is a topographymeasurement system, level sensor or height sensor. Such a tool may beintegrated in the lithographic apparatus, for measuring a topography ofa top surface of a substrate (or wafer). A map of the topography of thesubstrate, also referred to as height map, may be generated from thesemeasurements indicating a height of the substrate as a function of theposition on the substrate. This height map may subsequently be used tocorrect the position of the substrate during transfer of the pattern onthe substrate, in order to provide an aerial image of the patterningdevice in a properly focus position on the substrate. It will beunderstood that “height” in this context refers to a dimension broadlyout of the plane to the substrate (also referred to as Z-axis).Typically, the level or height sensor performs measurements at a fixedlocation (relative to its own optical system) and a relative movementbetween the substrate and the optical system of the level or heightsensor results in height measurements at locations across the substrate.

An example of a level or height sensor LS as known in the art isschematically shown in FIG. 5 , which illustrates only the principles ofoperation. In this example, the level sensor comprises an opticalsystem, which includes a projection unit LSP and a detection unit LSD.The projection unit LSP comprises a radiation source LSO providing abeam of radiation LSB which is imparted by a projection grating PGR ofthe projection unit LSP. The radiation source LSO may be, for example, anarrowband or broadband radiation source, such as a supercontinuum lightsource, polarized or non-polarized, pulsed or continuous, such as apolarized or non-polarized laser beam. The radiation source LSO mayinclude a plurality of radiation sources having different colors, orwavelength ranges, such as a plurality of LEDs. The radiation source LSOof the level sensor LS is not restricted to visible radiation, but mayadditionally or alternatively encompass UV and/or IR radiation and anyrange of wavelengths suitable to reflect from a surface of a substrate.

The projection grating PGR is a periodic grating comprising a periodicstructure resulting in a beam of radiation BE1 having a periodicallyvarying intensity. The beam of radiation BE1 with the periodicallyvarying intensity is directed towards a measurement location MLO on asubstrate W having an angle of incidence ANG with respect to an axisperpendicular (Z-axis) to the incident substrate surface between 0degrees and 90 degrees, typically between 70 degrees and 80 degrees. Atthe measurement location MLO, the patterned beam of radiation BE1 isreflected by the substrate W (indicated by arrows BE2) and directedtowards the detection unit LSD.

In order to determine the height level at the measurement location MLO,the level sensor further comprises a detection system comprising adetection grating DGR, a detector DET and a processing unit (not shown)for processing an output signal of the detector DET. The detectiongrating DGR may be identical to the projection grating PGR. The detectorDET produces a detector output signal indicative of the light received,for example indicative of the intensity of the light received, such as aphotodetector, or representative of a spatial distribution of theintensity received, such as a camera. The detector DET may comprise anycombination of one or more detector types.

By means of triangulation techniques, the height level at themeasurement location MLO can be determined. The detected height level istypically related to the signal strength as measured by the detectorDET, the signal strength having a periodicity that depends, amongstothers, on the design of the projection grating PGR and the (oblique)angle of incidence ANG.

The projection unit LSP and/or the detection unit LSD may includefurther optical elements, such as lenses and/or mirrors, along the pathof the patterned beam of radiation between the projection grating PGRand the detection grating DGR (not shown).

In an embodiment, the detection grating DGR may be omitted, and thedetector DET may be placed at the position where the detection gratingDGR is located. Such a configuration provides a more direct detection ofthe image of the projection grating PGR.

In order to cover the surface of the substrate W effectively, a levelsensor LS may be configured to project an array of measurement beams BE1onto the surface of the substrate W, thereby generating an array ofmeasurement areas MLO or spots covering a larger measurement range.

Various height sensors of a general type are disclosed for example inU.S. Pat. Nos. 7,265,364 and 7,646,471, both incorporated by reference.A height sensor using UV radiation instead of visible or infraredradiation is disclosed in US2010233600A1, incorporated by reference. InWO2016102127A1, incorporated by reference, a compact height sensor isdescribed which uses a multi-element detector to detect and recognizethe position of a grating image, without needing a detection grating.

Another type of metrology tool used in IC manufacture is an alignmentsensor. A critical aspect of performance of the lithographic apparatusis therefore the ability to place the applied pattern correctly andaccurately in relation to features laid down in previous layers (by thesame apparatus or a different lithographic apparatus). For this purpose,the substrate is provided with one or more sets of marks or targets.Each mark is a structure whose position can be measured at a later timeusing a position sensor, typically an optical position sensor. Theposition sensor may be referred to as “alignment sensor” and marks maybe referred to as “alignment marks”.

A lithographic apparatus may include one or more (e.g. a plurality of)alignment sensors by which positions of alignment marks provided on asubstrate can be measured accurately. Alignment (or position) sensorsmay use optical phenomena such as diffraction and interference to obtainposition information from alignment marks formed on the substrate. Anexample of an alignment sensor used in current lithographic apparatus isbased on a self-referencing interferometer as described in U.S. Pat. No.6,961,116. Various enhancements and modifications of the position sensorhave been developed, for example as disclosed in US2015261097A1. Thecontents of all of these publications are incorporated herein byreference.

FIG. 6 is a schematic block diagram of an embodiment of a knownalignment sensor AS, such as is described, for example, in U.S. Pat. No.6,961,116, and which is incorporated by reference. Radiation source RSOprovides a beam RB of radiation of one or more wavelengths, which isdiverted by diverting optics onto a mark, such as mark AM located onsubstrate W, as an illumination spot SP. In this example the divertingoptics comprises a spot mirror SM and an objective lens OL. Theillumination spot SP, by which the mark AM is illuminated, may beslightly smaller in diameter than the width of the mark itself.

Radiation diffracted by the mark AM is collimated (in this example viathe objective lens OL) into an information-carrying beam IB. The term“diffracted” is intended to include zero-order diffraction from the mark(which may be referred to as reflection). A self-referencinginterferometer SRI, e.g. of the type disclosed in U.S. Pat. No.6,961,116 mentioned above, interferes the beam D3 with itself afterwhich the beam is received by a photodetector PD. Additional optics (notshown) may be included to provide separate beams in case more than onewavelength is created by the radiation source RSO. The photodetector maybe a single element, or it may comprise a number of pixels, if desired.The photodetector may comprise a sensor array.

The diverting optics, which in this example comprises the spot mirrorSM, may also serve to block zero order radiation reflected from themark, so that the information-carrying beam D3 comprises only higherorder diffracted radiation from the mark AM (this is not essential tothe measurement, but improves signal to noise ratios).

Intensity signals SI are supplied to a processing unit PU. By acombination of optical processing in the block SRI and computationalprocessing in the unit PU, values for X- and Y-position on the substraterelative to a reference frame are output.

A single measurement of the type illustrated only fixes the position ofthe mark within a certain range corresponding to one pitch of the mark.Coarser measurement techniques are used in conjunction with this toidentify which period of a sine wave is the one containing the markedposition. The same process at coarser and/or finer levels may berepeated at different wavelengths for increased accuracy and/or forrobust detection of the mark irrespective of the materials from whichthe mark is made, and materials on and/or below which the mark isprovided. The wavelengths may be multiplexed and de-multiplexedoptically so as to be processed simultaneously, and/or they may bemultiplexed by time division or frequency division.

In this example, the alignment sensor and spot SP remain stationary,while it is the substrate W that moves. The alignment sensor can thus bemounted rigidly and accurately to a reference frame, while effectivelyscanning the mark AM in a direction opposite to the direction ofmovement of substrate W. The substrate W is controlled in this movementby its mounting on a substrate support and a substrate positioningsystem controlling the movement of the substrate support. A substratesupport position sensor (e.g. an interferometer) measures the positionof the substrate support (not shown). In an embodiment, one or more(alignment) marks are provided on the substrate support. A measurementof the position of the marks provided on the substrate support allowsthe position of the substrate support as determined by the positionsensor to be calibrated (e.g. relative to a frame to which the alignmentsystem is connected). A measurement of the position of the alignmentmarks provided on the substrate allows the position of the substraterelative to the substrate support to be determined.

For optical semiconductor metrology, inspection applications, such as inany of the aforementioned metrology tools, a bright light source whichoutputs coherent radiation, simultaneously covering a broad wavelengthrange (e.g., from UV to IR), is often preferred. Such a broadband lightsource can help improve the flexibility and robustness of applicationsby allowing wafers with different material characteristics to beoptically examined in the same setup/system without a need for anyhardware change (e.g., changing a light source so as to have a specificwavelength). Allowing the wavelength to be optimized for a specificapplication also means that the accuracy of measurements can be furtherincreased.

Gas lasers, which are based on the gas-discharge effect tosimultaneously emit multiple wavelengths, can be used in theseapplications. However, intrinsic issues such as high intensityinstability and low spatial incoherence associated with gas lasers canmake them unsuitable. Alternatively, outputs from multiple lasers (e.g.,solid-state lasers) with different wavelengths can be spatially combinedinto the optical path of a metrology or inspection system so as toprovide a multiple wavelength source. The complexity and highimplementation costs, which increases with the number of wavelengthsdesired, prevents such a solution from being widely used. In contrast, afiber-based broadband or white light laser, also called supercontinuumlaser, is able to emit radiation with high spatial coherence and broadspectral coverage, e.g., from UV to IR, and therefore is a veryattractive and practical option.

A hollow-core photonic crystal fiber (HC-PCF) is a special type ofoptical fiber that comprises a central hollow core region and an innercladding structure surrounding the hollow core, both of which extendaxially along the entire fiber. The light guidance mechanism is enabledthe inner cladding waveguide structure, which may comprise, for example,thin-walled glass elements. The radiation is thus confined predominantlyinside a hollow core and propagates along the fiber in the form oftransverse core modes.

A number of types of HC-PCFs can be engineered, each based on adifferent physical guidance mechanism Two such HC-PCFs include:hollow-core photonic bandgap fibers (HC-PBFs) and hollow-coreanti-resonant reflecting fibers (HC-ARFs).

Detail on the designing and manufacturing of HC-PCFs can be found inEuropean patent application EP3136143A1, which is incorporated herein byreference. HC-PBFs are configured to offer low loss but narrow bandwidthlight guidance via a photonic bandgap effect established by the claddingstructure surrounding the central hollow core. Whereas, HC-ARFs areengineered to significantly broaden the transmission bandwidth viaanti-resonant reflection of light from the cladding. HC-PCFs comprisehollow channels which are filled with a fluid, such that they possessresultant desired characteristics for various light guidingapplications; for example, high-power beam delivery using HC-PBFs andgas-based white light generation (or supercontinuum generation) usingHC-ARFs.

For gas-based white light generation, a HC-ARF is comprised within a gascell, which is designed to operate, for example, at a pressure up tomany 100s of bars (e.g., between 3-1000 bar). The gas-filled HC-ARF canact as an optical frequency converter when being pumped by an ultrashortpump laser pulse with sufficient peak power. The frequency conversionfrom ultrashort pump laser pulses to broadband laser pulses is enabledby a complicated interplay of the dispersion and nonlinear opticalprocesses inside the gas-filled fiber. The filling gas can be a noblegas such as Argon and Krypton, a Raman active gas such as Hydrogen andNitrogen, or a gas mixture. Depending on the type of filling gas, thenonlinear optical processes can include modulational instability,soliton fission, Kerr effect, Raman effects and dispersive wavegeneration, details of which are described in WO2018/127266A1 and U.S.Pat. No. 9,160,137B1 (both of which are hereby incorporated byreference). Since the dispersion of the filling gas can be tuned byvarying the gas cell pressure, the generated broadband pulse dynamicsand the associated spectral broadening characteristics can be adjustedso as to optimize the frequency conversion. The generated broadbandlaser output can cover wavelengths from UV (e.g., <200 nm) to mid-IR(e.g., >2000 nm).

As illustrated in FIG. 7 , an optical component 100, 100′, 100″comprises a HC-PCF (e.g., a HC-ARF) 101 having a specific fiber lengthand a gas cell 102 filled with a working gas or a gas mixture at aspecific pressure or with a pressure distribution. The gas cell 102further comprises an input optical window 103 a and an output opticalwindow 103 b, located at respective ends of the gas cell 102. The inputoptical window 103 a is operable to admit ultrashort pump laser pulsesinto the gas cell 102 via the window. After being coupled into thegas-filled HC-PCF 101, pump laser pulses (not shown) propagate along thefiber where they experience significant spectral broadening. Resultantbroadband laser pulses are subsequently discharged from the gas cell 102via the output window 103 b.

To fill the HC-PCF 101 with a working gas, the gas cell 102 should be incommunication with a pressurized gas supply or reservoir (not shown).The inner surfaces of the walls and windows 103 a, 103 b of the gas cell102 enclose a cavity with a volume V. The axis of the gas cell isparallel to the axis of the HC-PCF 101

FIGS. 7(a)-(c) schematically depicts three known configurations of theoptical component 100, 100′, 100″. FIG. 7(a) illustrates a firstconfiguration where the entire HC-PCF 101 is comprised within a singlegas cell 102. FIG. 7(b) illustrates an alternative arrangement where theentire HC-PCF 101 is comprised in several (e.g., three) sub-cells 102 a,102 b, 102 c which are interconnected by using appropriate an sealingmechanism 105. The pressure-tight connections ensure all the sub-cellsto reach the same pressure desired for white light generation. FIG. 7(c)illustrates another configuration where the two fiber ends 101 a, 101 cof the HC-PCF (101) are comprised in two separate gas cells 102 a,102 crespectively, while a central portion (101 b) of the fiber, acting as afluid connection, is comprised outside of the gas cells.

The gas cells of the prior art all have a relatively large internallocal cavity volume V_(loc), which is defined as a cavity volume perunit fiber length; for example, expressed in the unit of cm³ per cmfiber. It has been found that the performance of a white light laser canbe degraded by the presence of contaminating particles, which aregenerally proportional to the size of the local cavity volume. Thelarger the local cavity volume V_(loc) of a gas cell is, the more gasatoms or molecules are required to reach a specific pressure, andcorrespondingly the more contaminating particles would be present in thegas cell. Under the influence of high intensity light, the contaminatingparticles may react or interact with the material of the HC-PCF andthereby locally modify the structure of the HC-PCF. The morecontaminating particles are present in the gas cell, the more or largerthe local change may be. It is for example seen that contamination canaccumulate close to the light input or light output window of theHC-PCF. The accumulation of contamination can be reduced by ensuringthat the amount of contaminating particles is small within the gas cellas a whole. It is also to be noted that the gas may circulate throughthe gas cell under the influence of temperature different and/or diffusein and out of the HC-PCF. Furthermore, a gas cell is a pressurizedcontainer and imposes substantial requirements on safety. The potentialfor hazard of such a gas cell increases with the size of the localcavity volume V_(loc).

Therefore, to address these issues, an optical component is proposedwhich has a suitably dimensioned local cavity volume V_(loc). Byminimizing the local cavity volume V_(loc) of a gas cell of an opticalcomponent, it is possible to simultaneously improve the opticalperformance and relax operating safety requirements for a HC-PCF basedbroadband laser.

FIG. 8(a) illustrates a first such optical component 200, comprisingonly a single gas cell 202. In order to lower the local cavity volumeV_(loc), the gas cell is designed such that the spacing between theouter surface of the HC-PCF 201 and the inner surface of the gas cellwall 206 is reduced to a preferable range, e.g., 0-3 cm. This spacingcan be made sufficiently small in a number of ways. For example, the gascell may comprise thick walls. Alternatively, or in addition, an innersleeve or cylinder may be introduced within the volume to effectivelythicken the walls and ensure a smaller gas volume. Any arrangement toarrive at the claimed local cavity volume can be used. In addition, thefeature of thickening the cell walls and/or introducing an inner sleeveto achieve this can be done independently of arriving at the claimedlocal cavity volume (e.g., 36 cm³ or smaller)

The cavity volume V of the gas cell 202 comprises a central cavityvolume defined by the gas cell 202 along the length of the HC-PCF 201,(e.g., the cross-sectional area A of the central cavity multiplied bythe length L of the HC-PCF 201), and two end cavity volumes V_(end),each defined by the gas cell volume between an end of the HC-PCF 201 anda respective inner surface of the output window 203 a, 203 b. The lengthL of the HC-PCF 201 may lie, for example, in the range 0-1000 cm. Thecavity volume (204) can be calculated by:

V=V _(cen)+2V _(end),  [Equation 1]

where V _(cen) =A×L.  [Equation 2]

The local cavity volume V_(loc), at least along the length L of theHC-PCF 201, can then be obtained by:

$\begin{matrix}\begin{matrix}{V_{loc} = {V_{cen}/{L.}}} \\{\approx {V/{L\left( {{{assuming}{that}{}V_{end}} \ll V_{cen}} \right)}}}\end{matrix} & \left\lbrack {{Equation}3} \right\rbrack\end{matrix}$

It is proposed that this local cavity volume along the length of HC-PCF201 is at least below 36 cm³ per cm fiber (i.e., HC-PCF) length. Inother embodiments, the local cavity volume may be below 32 cm³ per cmfiber length, below 28 cm³ per cm fiber length, below 24 cm³ per cmfiber length, below 20 cm³ per cm fiber length, below 16 cm³ per cmfiber length, below 12 cm³ per cm fiber length, below 8 cm³ per cm fiberlength or below 4 cm³ per cm fiber length. The local cavity volume isdefined for at least 70%, 80%, 90%, 95% or 98% of the length of thehollow-core photonic crystal fiber which is enclosed within the gas cell(i.e., the volume is so limited to these values for at least theseproportions of the fiber length).

FIG. 8(b) shows a second embodiment of the invention. In thisembodiment, the optical component is comprised of multiple sub-cells,more specifically three sub-cells 202 a, 202 b, 202 c. The HC-PCF (201)is comprised in several sub-cells, including two end sub-cells 202 a,202 c with a larger external cross-section with respect to a centralsub-cell 202 b. The central sub-cell 202 b can be rigid or flexible. Forexample, the central sub-cell 202 b can comprise a conventional pressurepipe made from a certain material (e.g., stainless steel) or a mixtureof different materials (e.g., metal alloys or plastics). The two endsub-cells 202 a and 202 c can have the same external transversedimensions as those of the single gas cell 202 of FIG. 8(a). Whateverthe external dimensions, the internal dimensions, at least along thelength L of the HC-PCF 201, will be such that the local cavity volumealong length L is below 36 cm³ per cm fiber, and may comprise any of thevalues described above in relation to FIG. 8(a).

The second embodiment 200′ described above can be further simplified byremoving the central sub-cell 202 b to correspond with an arrangementsimilar to that illustrated in FIG. 7(c). FIG. 8(c) illustrates such anoptical component 200″, the two end sub-cells 202 d, 202 e may have thesame external and internal dimensions as those illustrated in FIG. 7(b).The two end portions of HC-PCF 201 are housed in respective ones ofthese two end sub-cells 202 d, 202 e while a central portion of theHC-PCF 201 is not enclosed in a sub-cell. Appropriate sealing mechanisms205 are applied to the gaps formed between the outer surface of thefiber and surfaces of the gas cell walls such that both sub-cells arepressure-tight and can be pressurized to a desired pressure. As before,the housed section of the fiber length (e.g., for the lengthcorresponding to sub-cells 202 d, 202 e have a maximum local cavityvolume of 36 cm³ per cm fiber, and optionally any of the valuesdescribed above in relation to FIG. 8(a).

Depending on the configuration of the optical component 200, thecross-section of the cavity, having a certain shape and an area, may beconstant along the fiber length, as in the case of the firstconfiguration. Alternatively, the cross-section of the cavity may belocation-dependent (or sub-cell dependent), as in the case of the secondconfiguration. The shape of the cross-section can be a rectangle, arounded rectangle, an ellipse or a circle

As mentioned above, the local cavity volume can be lowered by reducingthe spacing between the outer surface of the HC-PCF 201 and the innersurface of the gas cell wall 206. The reduction of the spacing can beachieved through several approaches, for example: 1) reducing thetransverse dimensions of the gas cell; 2) maintaining the existingtransverse external dimensions of the gas cell but increasing the wallthickness within the gas cell; 3) maintaining the existing transversedimensions of the gas cell and inserting filling elements within thecell cavity. Since the gas cells are typically manufactured beforeincorporating the HC-PCF 301, it is possible to use larger volume gascells (i.e., having a local cavity volume greater than 36 cm³ per cmfiber) and performing post-processing step thereon which lowers thelocal cavity volume. Such an optical component 300 is illustrated inFIG. 9 . In an embodiment, the post-processing step comprises theinsertion of filling elements 307 into the gas cell cavity 304. Thefilling elements can be solid blocks made from metal or other anothermaterial. Taking into account the gaps between the filing elements 307,the filing elements 307 should extend along most of the length of HC-PCF301, e.g., along at least 70%, 80%, 90%, 95% or 98% of the length ofHC-PCF 301. The basic principle of using filing elements 307 is alsoapplicable for the embodiments of FIGS. 8(b) and 8(c), i.e., for thesub-cells 202 a, 202 c 202 d, 202 e; e.g., to extend along most (e.g.,at least 70%, 80%, 90%, 95%, or 98%) of the length of a respectiveportion of HC-PCF 301 comprised within that cell.

The cross-section of any of the gas cells (or sub-cells) disclosedherein may have any suitable shape, for example an (e.g., rounded)rectangle, an ellipse or a circle.

The HC-PCF 301 as described herein may comprise any suitable fiberdevice such as, for example: a hollow-core anti-resonant reflectingfiber, a inhibited coupling hollow-core photonic crystal fiber, a hollowcore revolver fiber and a nested anti-resonant reflecting fiber.

A broadband light source device, e.g., being able to emit white light orbroadband pulses, may comprise an optical component as disclosed hereinand a pump laser operable to produce ultrashort pump laser pulses. Thepump laser pulses, generated by a pump laser device, can be nanosecondpulses, picosecond pulses or femtosecond pulses, and the pump wavelengthcan be in visible regime, near-IR regime or mid-IR regime. The pumplaser pulses can have a repetition frequency of several-hundred hertz(Hz), kilohertz (kHz), or megahertz (MHz).

A broadband light source device comprising any of the optical componentsdisclosed herein can be configured to output a broadband coherentradiation comprising, for example a wavelength range between 200 nm to2000 nm, or any sub-range within this range.

Potentially, by making the gas cell volume very small, this may enforcea laminar gas flow, which likely increases stability of the lightsource.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the clauses set out below.

-   1. An optical component, being configured to function as an optical    frequency converter in a broadband radiation source device,    comprising:-   a gas cell; and-   a hollow-core photonic crystal fiber at least partially enclosed    within the gas cell, and-   wherein a local cavity volume of the gas cell comprises a maximum    value of 36 cm³ per cm of length of the hollow-core photonic crystal    fiber.-   2. The optical component of clause 1, wherein the hollow-core    photonic crystal fiber is wholly enclosed within the gas cell.-   3. The optical component of clause 1, wherein the gas cell comprises    a plurality of sub-cells, the plurality of sub-cells comprising at    least two end sub-cells, enclosing respective ends of the    hollow-core photonic crystal fiber.-   4. The optical component of clause 3, wherein the sub-cells extend    along the full length of the hollow-core photonic crystal fiber and    further comprise at least one central sub-cell.-   5. The optical component of clause 4, wherein an external    cross-section diameter of the central sub-cell is smaller than an    external cross-section diameter of the two end sub-cells.-   6. The optical component of clause 4, wherein the central sub-cell    comprises a pressure pipe.-   7. The optical component of clause 1, wherein a maximum value of the    local cavity volume is one of: 32 cm³ per cm of length of the    hollow-core photonic crystal fiber, 28 cm³ per cm of length of the    hollow-core photonic crystal fiber, 24 cm³ per cm of length of the    hollow-core photonic crystal fiber, 20 cm³ per cm of length of the    hollow-core photonic crystal fiber, 16 cm³ per cm of length of the    hollow-core photonic crystal fiber, 12 cm³ per cm of length of the    hollow-core photonic crystal fiber, 8 cm³ per cm of length of the    hollow-core photonic crystal fiber, or 4 cm³ per cm of length of the    hollow-core photonic crystal fiber.-   8. The optical component of clause 1, wherein a gas cell comprises    filling elements to reduce a local cavity volume of the optical    component.-   9. The optical component of clause 1, wherein the hollow-core    photonic crystal fiber comprises one or more of: a hollow-core    anti-resonant reflecting fiber, an inhibited coupling hollow-core    photonic crystal fiber, a hollow core revolver fiber, and a nested    anti-resonant reflecting fiber.-   10. The optical component of clause 1, comprising an input optical    window for admitting a pump laser pulse and an output optical window    for outputting the broadband radiation, wherein the input optical    window and output optical window are located at respective ends of    the gas cell.-   11. The optical component of clause 1, wherein the local cavity    volume is defined for at least 70%, 80%, 90%, 95% or 98% of the    length of the hollow-core photonic crystal fiber that is enclosed    within the gas cell.-   12. A broadband light source device comprising an optical component    of clause 16, and being configured for creating broadband radiation.-   13. The broadband light source device of clause 12, further    comprising a pump laser being operable to generate ultrashort pump    laser pulses, the optical component being operable to convert the    ultrashort pump laser pulses into the broadband radiation.-   14. A metrology device comprising a broadband light source device of    clause 13.-   15. The metrology device of clause 14, comprising a scatterometer    metrology apparatus, a level sensor or an alignment sensor.

1. An optical component configured to function as an optical frequencyconverter in a broadband radiation source device, the optical componentcomprising: a gas cell; and a hollow-core photonic crystal fiber atleast partially enclosed within the gas cell, wherein the gas cellcomprises a plurality of sub-cells, the plurality of sub-cellscomprising: two end sub-cells enclosing respective ends of thehollow-core photonic crystal fiber; and a central sub-cell between thetwo end sub-cells.
 2. The optical component of claim 1, wherein theplurality of sub-cells extend along a portion of the length of thehollow-core photonic crystal fiber within the gas cell.
 3. The opticalcomponent of claim 1, wherein an external cross-section diameter of thecentral sub-cell is smaller than an external cross-section diameter ofthe two end sub-cells.
 4. The optical component of claim 1, wherein thecentral sub-cell is rigid.
 5. The optical component of claim 4, whereinthe central sub-cell comprises a pressure pipe.
 6. The optical componentof claim 1, wherein the central sub-cell is flexible.
 7. The opticalcomponent of claim 6, wherein the central sub-cell comprises plastics,metal alloys, or a combination thereof.
 8. The optical component ofclaim 1, wherein the gas cell comprises a structural integrity forwithstanding pressures of approximately 3 to 1000 bar.
 9. The opticalcomponent of claim 1, wherein the gas cell comprises filling elementsconfigured to reduce a local cavity volume of the optical component. 10.The optical component of claim 9, wherein the filling elements arearranged symmetrically about an optical axis of the hollow-core photoniccrystal fiber.
 11. The optical component of claim 9, wherein the fillingelements are arranged in the two end sub-cells.
 12. The opticalcomponent of claim 9, wherein the filling elements extend along acircumference of the gas cell.
 13. The optical component of claim 9,wherein the filling elements comprise a solid material.
 14. The opticalcomponent of claim 9, wherein the filling elements extend along a lengthof the gas cell within the gas cell.
 15. The optical component of claim9, wherein the filling elements comprise a rectangular cross-section, asquare cross-section, a rounded rectangular cross-section, a roundedsquare cross-section, an elliptical cross-section, a circularcross-section, or an annular cross-section.
 16. The optical component ofclaim 1, further comprising: an input optical window configured to admita pump laser pulse; and an output optical window configured to output abroadband radiation, wherein the input optical window and the outputoptical window are located at respective opposite ends of the gas cell.17. A broadband light source device comprising the optical component ofclaim 1, and being configured for creating broadband radiation.
 18. Thebroadband light source device of claim 17, further comprising a pumplaser being operable to generate ultrashort pump laser pulses, theoptical component being operable to convert the ultrashort pump laserpulses into the broadband radiation.
 19. A metrology device comprisingthe broadband light source device of claim
 18. 20. The metrology deviceof claim 19, comprising a scatterometer metrology apparatus, a levelsensor, or an alignment sensor.